The present invention relates to a process for producing a substrate member for a magnetic recording disc formed of ceramic materials. More particularly, it is concerned with a process for producing a substrate member for a magnetic recording disc provided thereon with a glass coating layer having substantially no air bubbles and strain due to machining as well as having an improved surface roughness.
In general, the substrate members (or base plates) for a magnetic recording disc is required to have the following properties:
(1) an excellent surface roughness after polishing so as to achieve stable floating of the magnetic head and stable recording characteristics in association with floating height of the magnetic head as low as 0.3 micron (.mu.m) or below;
(2) free from any projections and pinhole-like indents which are essentially attributable to the deficiency in the magnetic film formed on the surface of the substrate member;
(3) mechanical strength sufficient to withstand machining, polishing or high speed rotation during use; and
(4) sufficient corrosion-resistance, weather-resistance, and heat-resistance.
Heretofore, aluminum alloys have been used as the substrate members for magnetic recording discs. However, due to the crystal anisotropy in the aluminum alloy materials, defects in these materials as well as non-metallic inclusions existing therein, such non-metallic inclusions are apt to remain on the surface of the aluminum alloy substrate in the form of protrusions, or they tend to separate away from the surface thereof to give rise to indents, even after machining and polishing, with the consequence that a surface roughness (R.sub.z) of about 200 angstroms at the most could only be attained notwithstanding extensive polishing work having been carried out on it. Such surface state of the material having protrusions, indents and undulations is not satisfactory as the substrate for the high density magnetic recording disc.
That is to say, the machined quality of the surface of the magnetic disc substrate member directly affects the run-out and speed acceleration component caused by the magnetic recording disc, the signal errors of the magnetic recording medium, and so forth.
Since the aluminum alloys are metallic materials, they have a Vickers' hardness on the order of Hv 100 (in the case of ceramics, it is more than Hv 600) and a bending strength on the order of 1,000 kgf/cm.sup.2 (in the case of ceramics, it is more than 4,000 kgf/cm.sup.2) For that reason, as the recording density increases, more stringent requirements are imposed upon the shape and the dimensional precision thereof in respect of scratch, flaw, surface flatness and undulation, on account of which more difficulty is accompanied in its machining.
Also, in the case of the aluminum alloy substrate members, there would be a possibility of abrasive particles being packed in the indents at the surface part of the substrate member at the time of its machining by use of abrasive particles, which entails another problem; and, moreover, in order to increase the surface-corrosion-resistance and the weather-resistance as well as to prevent the substrate member from its surface contamination, a great deal of care should be taken to secure cleanliness and rust-prevention as well as to avoid contamination, etc. in the production steps of the substrate inclusive of the lathe-turning and the polishing works, as well as in the storage period thereof.
For the purpose of improving the aluminum alloy substrate members, there has so far been proposed a method, in which a film having a high hardness is formed on the surface thereof. As an example, there has been adopted a method, in which an alumite layer is formed on the surface of the aluminum alloy substrate to increase its hardness, thereby improving its abrasive machinability. However, traces of impurities (such as Fe, Mn, Si) contained in the aluminum alloys precipitate as intermetallic compounds during formation of the alumite, which are liable to bring about the surface indents after the alumite treatment.
It is extremely difficult to attempt further purification of the aluminum alloy matrix from the point of view of its production process. In addition, the aluminum alloys raise a handling problem from the standpoint of their corrosion-resistance and cleanness.
Furthermore, formation of a thin film magnetic recording medium by plating or sputtering onto the surface of the aluminum alloy poses problems in connection with the occurrence of chemical reactions and diffusion between the aluminum alloy and the magnetic thin film. There is also a problem such that, due to a heat treatment applied to the magnetic film upon its coating, deformation is caused to the aluminum alloy substrate, which would simultaneously bring about an increase in surface vibration and acceleration at the time of rotation of the substrate member (disc).
There has also been proposed a method, in which an oxide such SiO.sub.2, Al.sub.2 O.sub.3 or the like is formed on the aluminum alloy substrates by sputtering. This method, however, is disadvantageous in that the adhesion force between the aluminum alloy substrate and the sputtered oxide film is weak.
Alumina-based ceramic materials have become widely used in various fields due to their superiority over the aluminum alloy materials in respect of heat-resistance, wear-resistance, weather-resistance, insulation and mechanical strength. And, in order to fulfil the requirements for such magnetic disc substrate member, there is a strong demand on the alumina-based ceramic substrate member to have the surface and coated layer thereof free from any micropores and strains in association with the necessity for forming the thin film magnetic medium on the substrate member surface, and with the thinning and high densification of the recording medium.
In general, as the methods for producing the ceramic substrate member, there have been known the single-crystallization method, sintering methods wherein formed bodies (or compacts) are prepared by metal mold forming, rubber press forming, doctor blade forming, etc., and further, the hot-pressing (HP) process as well as the hot isostatic pressing (HIP) process have been known for obtaining the ceramic substrate having much more increased density. However, the single-crystallization method is not only high in production cost, but also it is difficult to produce a substrate having a large diameter. While, on the other hand, the hot isostatic pressing process and the hot pressing process are capable of producing a highly densified substrate, use of such ceramic substrate for a magnetic recording disc raises certain problems in its operational reliability such as the occurrence of drop-outs, head crush, and so on due to the minute surface defects (these methods are still liable to leave micropores of 5 um or below in the substrate) of the resulting substrate.
In general, the mechanochemical polishing method, which is applicable to the magnetic disc substrate member, etc. as the surface polishing method, has been known to be capable of precisely finishing the surface of silicon substrates, GGG crystals, ferrite, and so on without deteriorating the surface physical properties thereof. However, when this mechanochemical polishing method is applied to ceramic materials, in which micropores exist, it renders the micropores to be exposed to the surface of the ceramic material with the result that such ceramic materials are not eligible for magnetic disc substrate members, on which the thin film magnetic medium is to be coated. On the other hand, when an mechanochemical polishing method is applied to the alumina-based ceramic substrate, there arise problems such that exposure of micropores to the surface thereof and step-difference between crystal grains occur simultaneously due to the difference in the rate of chemical erosion on the surfaces of the constituent material or crystal grains.
With a view to solving, therefore, the disadvantages inherent in magnetic disc substrates composed of the ceramic materials, and to obtaining an improved ceramic type magnetic disc substrate member having excellent surface roughness and being substantially free from micropores and strains in its surface and coated layer, the present inventors have conducted strenuous studies and researche from various aspects, as the result of which they have developed a magnetic disc substrate member provided on its surface with a glass coating layer having excellent surface roughness and being substantially free from the micropores and strain due to machining, as the result of the mechanochemical polishing having been carried out on surface of the glass coating layer under particular conditions, after its coating.
However, of these glass coating layers formed on the surface of the ceramic surface by various methods, a glass coating layer formed on a disc having an outer diameter of 130 mm and an inner diameter of 40 mm, for example, contains about 100 pieces of air bubbles in the coated layer near the substrate surface, each having a diameter in a range of from 0.5 .mu.m to 5 .mu.m, which are sometimes exposed to the surface thereof. On account of this, when this glass-coated disc substrate member is covered with a magnetic recording medium to be made into a magnetic recording disc, such air bubbles are liable to be the cause of turning recording signals into error or noise signals with the consequence that operational reliability of the magnetic recording disc becomes poor upon its use. For the solution of such disadvantage, the air bubbles to be exposed as micropores to the surface of the glass coating layer and to be present in the coated layer itself should be reduced to 10 or less. Thus there is much desired in the art.